13. Threshold-tracking TMS without an MEP

Abstract

Introduction Transcranial magnetic stimulation (TMS) is a useful tool for quantifying cortical hyperexcitability in ALS. Short-interval intracortical inhibition (SICI) measures the suppression of descending corticomotoneuronal volleys by GABA-ergic inhibitory interneurons. Threshold-tracking TMS (tt-TMS) studies were developed to overcome the marked variability of motor evoked potentials (MEPs); tt-TMS “tracks” the stimulus intensity required to just evoke a small target MEP. Despite the success of studies using the tt-TMS technique, fasciculations can complicate recordings. Aims To explore the feasibility of using an accelerometer for the measurement of resting motor threshold (RMT), short interval intracortical inhibition (SICI) and intracortical facilitation (ICF). Methods Resting motor threshold (RMT), SICI and intracortical facilitation (ICF) were measured in 6 subjects (3M: 3F) using an accelerometer and compared to results recorded with a MEP. The MEP output was recorded over the thenar eminence and amplified (x1k, 2 Hz to 2 kHz) using a purpose-built amplifier. A natus reusable tremor sensory (6.3 mV/g) was fixed to the distal phalanx of digit 1, and its output was amplified with the same amplifier and settings. For both studies an output of 200 μ V was used as the target output measure. RMT in threshold-tracking studies is the magnetic stimulus intensity required to elicit the target MEP (or acceleration). SICI and ICF are measured using a pair of magnetic stimuli with the first stimulus subthreshold (70% of RMT) and the second stimulus tracking the target response. SICI was measured between 1 and 7 ms and ICF between 10 and 30 ms. Results A low threshold twitch was easily distinguished using an accelerometer, and was fairly insensitive to the orientation of the accelerometer on the thumb. RMT measured using the accelerometer was 56.5 ± 2.3% and 61.2 ± 2.1% for the MEP, though this was not statistically significant (p = 0.14). Similarly, average SICI (1–7 ms) was lower (11.2 ± 5.0%) when recorded using the accelerometer and 16.8 ± 6.1% for the MEP (p = 0.20). Average ICF(10–30 ms) recorded using the accelerometer was −2.2 ± 3.1% and for MEP 4.7 ± 1.0 (p = 0.02). Conclusion This pilot study shows that an accelerometer provides an easy to use alternative to MEPs for tt-TMS studies. This may be particularly useful early in MND when fasciculations are common and the diagnostic utility of tt-TMS is most beneficial.